U.S. patent application number 15/811358 was filed with the patent office on 2018-05-10 for fibre optic laser machining equipment for etching grooves forming incipient cracks.
This patent application is currently assigned to ROFIN-LASAG AG. The applicant listed for this patent is ROFIN-LASAG AG. Invention is credited to Ulrich DUERR, Bruno Frei, Rudolf Von Niederhaeusern.
Application Number | 20180126488 15/811358 |
Document ID | / |
Family ID | 42940851 |
Filed Date | 2018-05-10 |
United States Patent
Application |
20180126488 |
Kind Code |
A1 |
DUERR; Ulrich ; et
al. |
May 10, 2018 |
FIBRE OPTIC LASER MACHINING EQUIPMENT FOR ETCHING GROOVES FORMING
INCIPIENT CRACKS
Abstract
The laser machining equipment for etching grooves in a wall of a
mechanical part, in particular of a connecting rod for a spark
ignition engine, is provided with a fibre optic laser device and
arranged to supply laser pulses. The fibre optic laser device is
controlled so that said laser pulses have a peak power of more than
400 W and at least two times greater than the maximum mean power of
said laser device and in that the duration of said laser pulses is
below or within the nanosecond range (1 ns to 1000 ns). According
to a first embodiment, the fibre optic laser device is controlled
in a quasi continuous wave (QCW) mode. According to a second
preferred embodiment, the fibre optic laser device is controlled in
a Q-switch mode. The selected operating modes increase machining
efficiency and produce a groove with an optimum transverse profile,
particularly with a small mean radius of curvature at the bottom of
the groove which then allows precise subsequent fracturing of the
mechanical part with less force.
Inventors: |
DUERR; Ulrich;
(Allmendingen, CH) ; Von Niederhaeusern; Rudolf;
(Tschingel ob Gunten, CH) ; Frei; Bruno;
(Thierachern, CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ROFIN-LASAG AG |
Thun |
|
CH |
|
|
Assignee: |
ROFIN-LASAG AG
Thun
CH
|
Family ID: |
42940851 |
Appl. No.: |
15/811358 |
Filed: |
November 13, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13699392 |
Feb 28, 2013 |
|
|
|
PCT/EP2011/058254 |
May 20, 2011 |
|
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15811358 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B23K 26/0869 20130101;
B23K 26/0624 20151001; B23K 26/36 20130101; B23K 26/0676 20130101;
B23K 26/106 20130101; B23K 2101/006 20180801; B23K 2101/005
20180801; B41M 5/24 20130101; F16C 9/045 20130101; B23K 26/364
20151001 |
International
Class: |
B23K 26/0622 20060101
B23K026/0622 |
Foreign Application Data
Date |
Code |
Application Number |
May 25, 2010 |
EP |
10163787.4 |
Claims
1. A method of etching at least one groove in a lateral wall or
surface of a mechanical part by laser pulses supplied by a fiber
optic laser device, the groove defining an incipient crack for
subsequent fracturing of the mechanical part into at least two
pieces, the method comprising: controlling the fiber optic laser
device so that the laser pulses have a peak power of more than 400
W and at least two times greater than the mean maximum power of the
laser device, and wherein a duration of the laser pulses is below
or within the nanosecond range of 1 ns to 1000 ns.
2. The etching method according to claim 1, wherein the fiber optic
laser device operates in a quasi continuous wave mode.
3. The etching method according to claim 2, wherein the laser
pulses have a peak power of between 400 W and 3000 W.
4. The etching method according to claim 1, wherein the fiber optic
laser device is operated in a Q-switch mode.
5. The etching method according to claim 1, wherein the fiber optic
laser device includes a seed laser source and at least one fiber
optic amplifier medium supplying the laser pulses at an output.
6. The etching method according to claim 4, wherein the laser
device is controlled so that the duration of the laser pulses is
between 50 ns and 400 ns.
7. The etching method according to claim 4, wherein the fiber optic
laser device is controlled so as to supply the laser pulses with a
peak power of more than 1000 W.
8. The etching method according to claim 4, wherein the fiber optic
laser device is controlled so as to supply the laser pulses with a
frequency of between 10 kHz and 200 kHz.
9. The etching method according to claim 1, wherein a low mode
optical cable is provided between the laser device and a machining
head to which the laser pulses are supplied.
10. The etching method according to claim 1, wherein the mechanical
part is a connecting rod in a main aperture of which two
diametrically opposite grooves are simultaneously etched.
11. The etching method according to claim 4, wherein the laser
device is controlled so that the duration of the laser pulses is
between 50 ns and 400 ns.
12. The etching method according to claim 5, wherein the fiber
optic laser device is controlled so as to supply the laser pulses
with a peak power of more than 1000 W.
13. The etching method according to claim 5, wherein the fiber
optic laser device is controlled so as to supply the laser pulses
with a frequency of between 10 kHz and 200 kHz.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a divisional of U.S. application
Ser. No. 13/699,392, filed Feb. 28, 2013, which is based on
PCT/EP2011/058254 filed Apr. 20, 2011, and claims priority to
European Patent Application 10163787.4, filed May 25, 2010, the
entire contents of each of which being incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention concerns the field of laser machining
of grooves in a wall or a surface of a mechanical part to define
incipient cracks for fracturing said mechanical part into at least
two pieces. In particular, the machined mechanical part is a
connecting rod for a spark ignition engine. These connecting rods
have a main aperture and are initially formed of a single part. Two
diametrically opposite grooves are etched into the circular lateral
wall of the main aperture. Then, using mechanical means, the
connecting rod is fractured into two pieces by pressure. This
technique is well known to those skilled in the art.
[0003] The use of a laser beam for etching the grooves forming
incipient cracks has numerous advantages. In particular, laser
technology can be used to make a relatively narrow and deep slot
resulting in a clean fracture along a plane containing the central
geometrical axis of the main aperture of the connecting rod.
BACKGROUND OF THE INVENTION
[0004] A fibre optic laser machining equipment is known, in
particular from DE Patent No. 10 2007 053 814. As mentioned in
paragraphs 3 and 4 of that document, various types of laser have
been used for the specific application concerned, but the profile
of the grooves obtained is not optimum. The quality of the machined
grooves is a decisive factor for obtaining a clean fracture along a
determined geometrical plane and also for fracturing with less
force. The aforementioned German Patent document proposes the use
of a particular type of laser device in the machining equipment,
namely a fibre optic laser. Preferably, the fibre optic laser
includes a set of diodes for pumping the active medium (doped
fibre). The use of a diode pumping means increases the frequency of
the pulses supplied by the laser device in the proposed pulsed
mode. According to paragraph 31 of DE Patent No. 10 2007 053 814,
the set of diodes forming the pumping means operates in a pulsed
manner. The frequency of the laser pulses produced may be located
within a working range of between 10 and 100 kHz. This prior art
document therefore proposes using a fibre optic laser operating in
pulsed mode, i.e. a mode where the pumping means is powered in a
pulsed manner to modulate the laser beam generated at the laser
device output. Thus, the pulsed mode consists in modulating the
continuous power (CW) of the laser so that the maximum pulse power
is equal to the CW of the laser (maximum mean power).
[0005] The laser device proposed in DE Patent No. 10 2007 053 814
cannot produce grooves with a sufficiently optimum profile. The use
of a fibre optic laser is an advantageous solution for obtaining a
good quality laser beam, required for machining a narrow, deep
groove. Indeed, a fibre optic laser can produce a high quality
laser beam which does not deteriorate during propagation in a low
mode fibre optic (preferably single mode) to the machining head of
the laser machining equipment. However, the pulsed mode proposed in
the aforementioned Patent document greatly limits the potential
resulting from the use of a fibre optic laser, in particular as
regards the depth/width ratio of the groove, the metallurgic
quality of the walls and the radius at the bottom of the
groove.
[0006] It is an object of the present invention to improve the
method of machining grooves forming incipient cracks by proposing a
laser equipment that can decrease stray thermal stresses which have
several damaging consequences for forming a groove and in the area
surrounding said groove (such as the transformation of the
metallurgic structure in both walls of the groove and the
appearance of micro-fissures in the walls). It is another object of
the present invention to increase the efficiency of machining
grooves in the lateral wall of connecting rods by allowing the use
of a simplified equipment and/or an equipment that can increase
machining speed and/or limit the movements of the machining head
for machining two diametrically opposite grooves in the main
aperture of a connecting rod.
SUMMARY OF THE INVENTION
[0007] In developing the laser machining equipment according to the
present invention, the inventors observed that a fibre optic laser
device operating in pulsed mode could not provide a groove with a
profile having a small ratio between the width and depth of the
groove as well as a small radius at the bottom of the groove, which
is a decisive factor for decreasing the force required to fracture
the mechanical part and also to ensure a fracture in the defined
geometrical plane. The disadvantage of the laser device proposed in
the aforementioned prior art document mainly arises from two
drawbacks of the proposed operating mode. First of all, the
proposed pulsed mode only modulates the generated laser beam to
supply pulses with a maximum power or peak power equal to the CW
that the laser can supply. In the aforecited prior art document,
the pulse power is comprised between 10 and 100 Wafts (W), and
preferably between 40 and 60 W. For large connecting rods, it is
proposed to use more powerful lasers supplying a peak power of
around 200 W. (It should be noted that higher power CW lasers exist
but they are not economically viable in the industry). It will be
noted that in a pulsed mode such as that proposed in the aforecited
prior art document, very powerful lasers, at the limits of current
industrial laser technology, have to be used to obtain pulses with
a peak power of around 200 W.
[0008] Laser pulses with a maximum power substantially equal to 200
W or less, with a relatively long pulse duration for providing
sufficient energy for the ablation of material, mostly cause the
material forming the machined mechanical part to melt. This melted
material causes a problem as regards obtaining a good quality
groove. In particular, the material has to be evacuated using a
high-pressure gas jet. The melted material also causes a problem as
regards the cleanness of the groove and the machined surface, and
the protective glass at the machining head output. Moreover, the
melted material and the ejection thereof using pressurised gas
limits the reduction in the radius of curvature at the bottom of
the groove despite the high quality of the laser beam produced by
the fibre optic laser equipment.
[0009] Next, another problem caused by the operating mode provided
in the aforecited document arises from the fact that the duration
of the pulses supplied is generally within the microsecond range
(.mu.s), i.e. more than one microsecond. In particular, diodes
operating in pulsed mode can supply pulses with a duration of
between 5 and 10 microseconds. Contrary to the statement of the
aforementioned document, an increase in frequency does not
necessarily cause a decrease in the quantity of energy per pulse.
The quantity of energy contained in each laser pulse is determined
by both the power and the duration of said pulse. A high frequency
certainly generally leads to a decrease in pulse duration for a
given mean power, but the pulsed mode proposed does not allow the
pulse duration to be decreased below 1 .mu.s in a conventional
equipment; which causes a problem of negative secondary thermal
effects. Indeed, the diffusion of heat in the machined material
depends upon the pulse duration. The longer the pulse duration, the
greater the secondary thermal effects will be, and particularly the
more the propagation of thermal energy in the area of the machined
groove will increase. This results first of all in an increase in
the melted material, which leads to a groove with a larger width
and a relative large mean radius of curvature at the bottom of the
groove. Thus, although the total energy per pulse is substantially
correct with a power of around 100 W and a pulse duration of
several microseconds, the profile of the groove obtaineds not
optimum.
[0010] In developing the invention, the inventors demonstrated that
a substantial increase in the peak power of the laser pulses
supplied decreases the quantity of melted material by increasing
the quantity of sublimed material. Moreover, by increasing the
pulse power, it is possible to supply the energy required per pulse
with a shorter pulse duration; which decreases the thermal stresses
at the periphery of the groove and also decreases the radius of
curvature of the bottom thereof.
[0011] The present invention therefore concerns a laser machining
equipment for etching at least one groove each defining an
incipient crack in a lateral wall or surface of a mechanical part,
said laser machining equipment being characterized by a fibre optic
laser device which is controlled so that the laser pulses supplied
have a peak power of more than 400 Watts and at least two times
higher than the mean maximum power of the fibre optic laser device
used and so that the duration therefore is within the nanosecond
(ns) range, i.e. comprised between one and a thousand nanoseconds.
Typically, the pulse duration is comprised between 50 ns and 400
ns.
[0012] According to a particular embodiment, the fibre optic laser
device is controlled in a quasi continuous wave (QCW) mode. A fibre
optic laser device controlled in QCW mode can for example produce
pulses with a maximum power or peak power ten times higher than the
mean maximum power of the laser device.
[0013] According to a preferred embodiment, the fibre optic laser
device is controlled in a Q switch mode. According to another
preferred embodiment, the fibre optic laser device includes a seed
laser source (for example a diode supplying pulses within the
nanosecond range) and at least one fibre optic amplifier medium
supplying the laser machining pulses abutput.
[0014] As a result of the features of the invention, it is possible
to machine very narrow and relatively deep grooves with a very
small radius at the bottom of the groove. The laser pulses supplied
by the fibre optic laser device according to the present invention
decrease the quantity of melted material and also significantly
limit the stray thermal effects responsible for the deterioration
in quality of the grooves obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The present invention will be described in more detail in
the following description, made with reference to the annexed
drawings, given by way of non-limiting example, and in which:
[0016] FIG. 1 is a schematic view of a connecting rod and a fibre
optic laser device associated withal groove machining head;
[0017] FIG. 2A shows a schematic perspective view of a groove
(partial view) machined using a prior art device.
[0018] FIG. 2B is a similar schematic view to that of FIG. 2A but
showing a groove obtained using a laser equipment according the
present invention.
[0019] FIG. 3 is a schematic cross-section of a particular first
embodiment of a machining head of a laser equipment according to
the present invention.
[0020] FIG. 4 is a schematic cross-section of a particular second
embodiment of a machining head of a laser equipment according to
the present invention.
[0021] FIG. 5 is a schematic cross-section of a particular third
embodiment of a machining head of a laser equipment according to
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] FIG. 1 is a schematic view of a laser machining equipment 2
for making grooves 8 and 9 in the lateral wall of the main aperture
6 of a connecting rod 4. These grooves are oriented along the
central geometric axis of the main aperture. The equipment includes
a fibre optic laser device 12 connected to a machining head 22 by a
flexible optical cable 24. Machining head 22 and connecting rod 4
are associated with a motorised means (not shown) of relative
movement along said central geometric axis for etching the grooves.
Laser device 12 include an active fibre optic medium 14 known to
those skilled in the art and a means 16 of pumping formed of optic
diodes coupled to the active medium. This device includes a control
unit 18 which controls the powering of the optical pumping means
and other parameters according to the selected operating mode.
Generally, a series of laser pulses is supplied.
[0023] The use of a fibre optic laser has several advantages
relating to the quality of the laser beam obtained. Further, the
beam can be brought to the machining head by a low mode optical
cable while still preserving good optical beam quality, which
simplifies the equipment. Beam quality is important to allow proper
focusing (even with an incident laser beam on the optical focusing
system with a relatively small diameter) and thus to reduce the
diameter of the beam at the focal point. This must enable narrow
grooves to be formed. However, making a narrow and sufficiently
deep groove, with walls defining an acute angle and a small radius
at the bottom of the groove, involves parameters other than the
beam quality. As mentioned above, controlling the supply of energy
and in particular controlling luminous intensity, i.e. the power
density, are decisive factors in machining this type of groove with
an optimal profile. The manner in which material is ablated in the
wall of the connecting rod is essential in achieving this optimum
profile.
[0024] The use of a fibre optic laser device operating in pulsed
mode by modulating the pumping power of the active medium, as
proposed in the prior art, results in pulses having a peak power
equal to the nominal power of the laser, which is generally less
than 200 W for an industrial fibre optic laser, and having a
duration of more than 1 .mu.s. This relatively low power cannot
provide sufficient luminous intensity to prevent a large part of
the material receiving the laser pulse from melting and thus
changing into a liquid state. The melted liquid material causes a
problem of evacuation and tends to remain partly at the bottom of
the groove. This results in a relatively large mean radius of
curvature R1 at the bottom of the groove, as is shown schematically
in FIG. 2A. Further, the relatively long duration of the pulses
also generates secondary thermal effects or thermal stresses in the
material, as the thermal energy propagates further into the
peripheral region 26 of the machined groove 28. Therefore, the
melted material, the quantity of energy provided by each laser
pulse and the duration of each pulse all contribute to widening the
groove and producing a relatively large mean radius R1 at the
bottom of the groove.
[0025] The findings brought to light in developing the invention
result in the selection of a particular control of the fibre optic
laser device. According to the invention, the fibre optic laser
device is controlled so that the laser pulses have a peak power of
more than 400 W and at least two times greater than the maximum
mean power of the laser device and so that the duration of the
laser pulses is within the nanosecond range (ns), i.e. between 1 ns
and 1000 ns or below.
[0026] According to a first operating mode of the fibre optic laser
device according to the present invention, this laser device is
controlled in a quasi continuous wave (QCW) mode. For a laser with
a power of between 50 W and 150 W, it is easy to obtain pulses with
a peak power of around 1000 W (1 kW). Depending upon the variant
and the application, the laser device is arranged to obtain a peak
power or maximum pulse power of between 400 W and around 3000 W (3
kW). Those skilled in the art of fibre optic lasers know how to
implement QCW mode and the specific diodes required to obtain such
laser pulses.
[0027] To obtain short pulses within the nanosecond range, in
particular between 50 ns and 400 ns, with very high power peaks,
two main variants described below were envisaged.
[0028] According to a second operating mode, the fibre optic laser
device is controlled in a Q-Switch mode. This second operating mode
is preferred since it advantageously obtains significant shorter
pulse durations than the QCW mode of the first operating mode
proposed and also much higher peak powers, for example of around 10
kW. It is therefore possible to obtain very high luminous
intensities for sublimating the material of the machined mechanical
C part, i.e. to change directly from a solid state to a gaseous
state. For example, the fibre optic laser device is controlled so
as to supply the laser pulses with a frequency of between 10 kHz
and 200 kHz. Since the duration of the pulses is very short, the
quantity of energy supplied per pulse is also limited. This
quantity of energy may be adjusted to optimise the laser machining
method according to the present invention, particularly between 0.1
mJ and 2 mJ. Since the duration of the pulses is very short, the
secondary thermal effects and penetration of thermal energy into
the material is greatly limited. This allows a very narrow and
relative deep groove to be obtained with an optimal profile, as
shown schematically in FIG. 2B. The perforations made are narrower
than those obtained in the prior art. The width/depth ratio of the
machined groove 30 is lower than that obtained with a prior art
laser device and the mean radius of curvature R2 at the bottom of
the groove is significantly lower than that (R1) of FIG. 2A. This
all results in a clean groove with a minimum of material ejected
onto the wall of the opening at the edge of the groove and also in
an improved incipient crack for the subsequent fracturing of the
mechanical part into two pieces.
[0029] According to a third embodiment which is also preferred, the
fibre optic laser device includes a seed laser source and at least
one fibre optic amplifier medium supplying the laser machining
pulses at output. The seed laser pulses form low power pulses that
can be produced with a very short duration and at a very high
frequency, for example at 10 MHz. These seed pulses are introduced
into the input of the fibre optic amplifier medium which
substantially maintains the duration and also the frequency of the
seed pulses and which greatly amplifies the pulse power. This means
of amplification can easily obtain peak powers of more than 1000 W.
Those skilled in the art know how to construct this type of fibre
optic laser device.
[0030] The machining method according to the invention and the
laser machining equipment for implementing said method have further
advantages. First of all, the generation of very high power pulses
makes it possible to envisage simultaneously machining two
diametrically opposite grooves in a connecting rod, in particular
by dividing the energy from each primary laser pulse into two
secondary pulses, the power of which is half that of said primary
laser pulse, while keeping the other benefits of the invention. A
particular machining head shown in FIG. 3 is a particular
embodiment using this additional advantage. Secondly, since the
melted material is greatly limited or eliminated in the laser
equipment according to the invention, the use of a high pressure
gas as in the prior art is no longer necessary. Thus, there is no
longer a requirement to use nozzles with a small orifice for
injecting the high pressure gas at the place of impact of the laser
beam onto the wall of the machined mechanical part. Gas may,
however, continue to be used in order to keep the machining head
clean, but this gas may be a low pressure gas and extend over a
wider area. Two particular machining heads shown respectively in
FIGS. 4 and 5 are embodiments which benefit from this additional
advantage.
[0031] Machining head 32 shown in FIG. 3 is connected to optical
cable 24 and receives at input a laser beam 34 formed of laser
pulses according to the invention. The input of this machining head
further includes a collimator 44 for the laser beam exiting the
fibre optic with a large aperture, a first semi-transparent mirror
used to divide primary beam 34 into two secondary beams 40 and 42
and a second mirror 38 for reflecting secondary beam 42 in a
substantially axial direction. Each of the two secondary beams is
associated with an optical focusing means schematically represented
by a convex lens 48 and 50 respectively, to focus these two
secondary beams on the lateral wall of connecting rod 4. To adjust
the respective focal points of the two secondary beams, lenses 48
and 50 may preferably be moved vertically. It will be noted that,
owing to the high quality of the laser beam produced by the fibre
optic laser, very good focussing can be obtained with an incident
laser beam on the relatively small diameter focussing means. Thus,
the convex lenses may have a relatively small diameter and the head
can maintain a compact form. The end portion 52 of the machining
head introduced into the aperture in connecting rod 4 includes a
mirror 54 with two inclined reflective surfaces for deviating the
two secondary beams in a substantially perpendicular direction to
the lateral surface of the aperture of the connecting rod and
respectively in two opposite directions for simultaneously
machining two diametrically opposite grooves 8 and 9. The two
secondary laser beams exit the end portion 52 respectively through
two diametrically opposite end apertures, defined by the orifices
of two nozzles 56 and 58, propagating in the same geometrical
plane. In a variant, the reflective surfaces of the prismatic
mirror 54 each have a inclination which reflects the secondary
incident beam obliquely to the machined lateral surface. In a known
manner, a gas may be provided inside the machining head and exit
through these two conventional nozzles. A single relative vertical
movement between connecting rod 4 and machining head 32 enables the
two grooves 8 and 9 to be machined in parallel.
[0032] Machining head 60 shown in FIG. 4 is connected to optical
cable 24 and receives at input a laser beam 34 formed of laser
pulses according to the invention. The input of this machining head
includes a collimator 66 for the laser beam exiting the fibre optic
with a large aperture, a removable mirror 62 associated with
motorised means 64 allowing a linear movement of said mirror 62
which reflects the beam in a parallel direction to the longitudinal
axis of the machining head, and a focusing means 68 (represented by
a convex lens) integral with the removable mirror. It will be noted
that in a variant, the focusing means is advantageously arranged
after the removable mirror, closer to the machined lateral surface.
Preferably, the focusing means can be moved relative to mirror 62
to adjust the position of the focal point. Finally, the beam ends
up on an inclined mirror 70 arranged at the end of the machining
head. This mirror 70 is oriented so that the plane of incidence of
the laser beam is parallel to the direction of movement of mirror
62. The beam reflected by mirror 70 exits the head through an end
slot 72 the height of which is at least equal to the length of
groove 8, i.e. equal to or slightly greater than the height of the
connecting rod. By moving, removable mirror 62 causes the laser
beam to make a vertical sweep of the wall of the aperture of
connecting rod 4. Thus the beam gradually moves along end slot 72
to machine groove 8 without any relative vertical movement between
machining head 60 and connecting rod 4. The machining head is
therefore held in a fixed position during the laser machining of a
groove. The end slot does not cause any particular problem here
since no high pressure gas is used for evacuating melted material.
However, a low pressure gas may be injected through slot 72 to
protect the protective glass arranged in the vicinity of said slot.
In a variant, a gas jet is injected between the wall of the
connecting rod and the end slot from above or below depending on
the direction of machining. The machining head is formed in two
parts 60A and 60b, the top part 60A being fixed (or able to move in
a horizontal linear direction) and the bottom part 60B being able
to rotate to allow the diametrically opposite groove to be machined
without having to rotate the top part connected to the optical
cable. To rotate the bottom part, a torque motor 76 is arranged
with the stator part 78 thereof connected to the fixed part 60A and
the rotor part 79 thereof connected to the rotating part 60B. This
motor has a central opening for the laser beam to pass through and
is actuated by a programmable control unit 80 used to provide it
with electric power.
[0033] FIG. 5 shows another embodiment of a machining head 82
according to the invention. This head comprises a top part 82A
which can undergo a vertical movement and a bottom part 82B which
can also undergo a rotation. A torque motor 76 described above is
arranged for rotating the bottom part so as to allow two
diametrically opposite grooves to be machined without having to 3o
rotate the top part 82A connected to optical cable 24. At the
optical cable output there is arranged a means 66 of collimating
the incoming laser beam, which is incident on a mirror 84 arranged
obliquely to reflect the laser beam 34 in an axial direction. The
bottom part 82B includes an objective lens 86 which can be moved
vertically to adjust the focal point, a first oblique mirror 88 and
a second mirror 89 finally reflecting the beam obliquely. The exit
angle of the laser beam can be varied according to the orientation
of mirrors 88 and 89. This particular arrangement can produce two
diametrically opposite grooves in small apertures, without having
to move the top part 82A of the machining head. Thus, the optical
axis of this top part merges with the central axis of the aperture
of mechanical part 84. A specific bottom part can be provided for
each different diameter. Preferably, the vertical position of lens
86 can be adjusted in order to adjust the focal point according to
the diameter of the connecting rod. It will be noted that the
bottom part 82B of the machining head is located entirely above the
part to be machined 84. This is made possible by the fact that the
method according to the invention does not require a gas for
ejecting melted material in the groove being formed and the
proportional quality factor in M.sup.2 is sufficiently small to
have a small focal point relatively far away from the focusing
means to make a groove. There is either a relative vertical
movement between part 84 and the machining head 82 or a scan is
made by varying the position of at least one of the two mirrors 88
and 89.
* * * * *